crplyr: A 'dplyr' Interface for Crunch

In order to facilitate analysis of datasets hosted on the Crunch
data platform <http://crunch.io/>, the 'crplyr' package implements 'dplyr'
methods on top of the Crunch backend. The usual methods 'select', 'filter',
'group_by', 'summarize', and 'collect' are implemented in such a way as to
perform as much computation on the server and pull as little data locally
as possible.